1. Field of the Invention
The present invention relates to a memory cell of a nonvolatile semiconductor memory in which a charge storage layer is constituted from an insulating film.
2. Description of the Related Art
One of the candidates for architecture of a memory cell of a NAND type flash memory whose gate length is 30 nm or less is a MONOS (metal/oxide film/nitride film/oxide film/semiconductor) structure with interferences between adjacent cells reduced.
The MONOS type memory cell has a characteristic in that a charge storage layer is constituted from an insulating film.
That is, the memory cell is constituted in such a way that, for instance, on a channel between two source/drain diffusion layers on an Si substrate, a tunnel insulating film through which electric charges are passed at the time of write/erase, a silicon nitride film functioning as the charge storage layer, a block insulating film having a function to block current, and a gate electrode are stacked.
Here, write/erase property and retention property are improved when the tunnel insulating film of the MONOS type memory cell is formed of a silicon oxynitride film (SiON) (for instance, refer to Jpn. Pat. Appln. KOKAI Publication No. 2004-165553).
Further, when changing the charge storage layer from the silicon nitride film to a high-k insulating film, it is possible to realize both increase in a trap density of the charge storage layer and decrease in Equivalent Oxide Thickness (EOT) simultaneously (for instance, refer to T. Sugizaki, M. Kobayashi, M. Ishida, H. Minakata, M. Yamaguchi, Y. Tamura, Y. Sugiyama, T. Nakanishi, and H. Tanaka, “Novel Multi-bit SONOS Type Flash Memory Using a High-k Charge Trapping Layer,” Symp. VLSI Tech. Digest p. 27 (2003)).
Furthermore, proposed is the technique in which both the tunnel insulating film and the charge storage layer are formed of the high-k insulating film (for instance, refer to Jpn. Pat. Appln. KOKAI Publication No. 2005-268756).
However, MONOS type memory cells having further superior property are required with respect to both write/erase and retention.